[Amps] a question on non-linearity

[CLIVE COLLINS]

Good evening Gentlemen,
Could some learned reader advise me as to what exactly a triode's characteristics would be like if it were to be described as "non-linear".
I have a working knowlege of the vacuum tube family but I am unsure as to what this description means.
I have something that will possibly make 5 kilowatts called a 7C23 by ITT and I cannot find any details other than as stated.
 
And yes, I do have a licence, but even the older members have mental block at times!
 
Regards and thanks in advance.
 
Clive GW3WEQ